1. Field of the Invention
The present invention relates to a stereoscopic-vision endoscope enabling stereoscopic observation of an object.
2. Description of the Related Art
In resent years, it has prevailed in the field of surgery to adopt an endoscopic approach instead of laparotomy. Specifically, the inside of an abdomen is observed by creating a small orifice in the abdomen and then inserting a rigid endoscope through the orifice. If necessary, the rigid endoscope is used in combination with a TV camera, so that a therapeutic instrument can be advanced for the purpose of surgery while viewing a monitor.
The rigid endoscope is usually used to provide a view of the inside of a body cavity as a plane image not giving depth perception. It is therefore difficult to observe fine irregularity on the surfaces of inner walls of a body cavity. Since depth information is unavailable, it takes too much time to complete surgery. For solving this problem, a stereoscopic-vision endoscope offering depth information has been developed recently.
The stereoscopic-vision endoscope is classified into two types. One of the types has two optical systems arranged in parallel with each other. Images provided by the optical systems are formed on imaging devices or the like. This type is referred to as a dual-relay type. The other type of stereoscopic-vision endoscope has an optical system at the distal end thereof. A light beam is divided at the position of exit-pupil formation. Images having parallax are formed on imaging devices or the like. This type is referred to as a pupil-division type.
In the dual-relay type endoscope, each of the optical systems has a large number of lenses. It is therefore hard to minimizing a difference between images provided by two optical systems. As far as the stereoscopic-vision rigid endoscope is concerned, the pupil-division type is more effective than the dual-relay type.
With the spread of such a stereoscopic-vision rigid endoscope, there is a growing need for a stereoscopic-vision rigid endoscope to be used for observation of fine regions including the brain and the inside of an eye. For observing the fine regions including the brain and the inside of an eye, it is a must that the stereoscopic-vision rigid endoscope has a thin insertional part.
Described in Germany Patent No. 9302898.9 is regarded as a conventional endoscope of the pupil-division type enabling stereoscopic visioning and having two optical systems. This endoscope is, as shown in FIG. 36, composed of an objective lens 121, an image transmission optical system 122, an image formation optical system 123, and a main objective lens 124, which share a common optical axis.
In this conventional endoscope, the main objective lens 124 is a system of lenses for forming an intermediate image I1 by converging light to infinity. An aperture stop 125 having two apertures is placed behind (at the position of an exit pupil of) the main objective lens 124. Beams confined by the aperture stop 125 pass through imaging optical systems 126a and 126b, and form images on imaging devices 127a and 127b, whereby a three-dimensional image is produced.
It is also described in the Germany patent publication that if the image transmission optical system 122 is realized with a refractive index distribution type lens, a thinner endoscope can be realized.
The aforesaid conventional endoscope has not been described to have the configuration permitting a proper sense of three-dimensionality and proper brightness which are required for stereoscopic visioning while realizing a thinner endoscope.
Another optical system for a conventional endoscope enabling stereoscopic visioning has been described in Japanese Patent Laid-Open No. 6-59199 shown in FIG. 37. This optical system is a single optical system composed of an objective optical system 200 and a relay optical system 201 which are axially symmetric. An image formed by the objective optical system 200 is transmitted by a predetermined distance by the relay optical system 201. A prism 202 is placed at the back end of the relay optical system 201, whereby an exit pupil is spatially divided into two portions so that a pair of right and left images having parallax can be picked up by imaging means 203 and 204 such as CCDs. The pair of right and left images thus picked up is converted into electric signals and displayed on a TV monitor that is not shown. At this time, when the right and left images to be displayed are switched at a high speed and shutter glasses whose movements are synchronous with the switching are employed, a right-eye image is viewed by a right eye and a left-eye image is viewed by a left eye. This results in stereoscopic visioning.
These types of objective and relay optical systems have the same structure as those employed in any conventional endoscope that is not designed for stereoscopic visioning. Many components contribute to formation of both the right and left light paths. Owing to the limited number of components, manufacturing errors are limited and assembling efficiency is excellent.
As for the stereoscopic-vision endoscope described in the Japanese Patent Laid-Open No. 6-59199, the degree of parallax is too low to provide a sufficient sense of three-dimensionality. This point will be described in conjunction with FIG. 38.
An image I' formed by the objective optical system 200 is transmitted by the relay optical system 201. Thereafter, a beam is divided into two portions by a pupil dividing means 206. Images are then formed on imaging devices 208a and 208b by means of right and left image formation optical systems 207a and 207b. The pupil dividing means 206 is composed of a pupil formation lens 210 for receiving a light beam emanating from a final image Ie of the relay optical system and then converging the light beam to infinity, a pupil dividing stop 211 for limiting the light beam and dividing it into two portions, and prisms 212a and 212b for extending a space between right and left light beams.
The pupil dividing stop 211 lies substantially at the position of an exit pupil and has apertures 211a and 211b at positions substantially mutually symmetric with respect to the optical axis of the relay optical system 201. Only the light beams (hatched areas in FIG. 38) passing through the apertures 211a and 211b contribute to image formation, and form two exit pupils as images provided by the objective optical system 200.
The relay optical system 201 includes one system of relay lenses or a plurality of systems of relay lenses. Each system of relay lenses is an afocal optical system that has lenses, which have substantially the same focal length and structure, mutually coupled with their focal points aligned and that offers a zero power.
The degree of parallax that is a main factor dominating a sense of three-dimensionality is expressed using an angle of introversion .alpha..
Assuming that a numerical aperture in the object space determined by the objective optical system 200 and relay optical system 201 is NAo, the diameter of an incident image is a, and a spacing between centers of gravity of two entrance pupils of the pupil dividing stop 211 is b, the angle of introversion .alpha. is expressed as follows: EQU .alpha.=2k.multidot.NAo (rad) (1)
where k denotes a pupil dividing ratio and equals to a quotient of b/a, and NAo=sin .theta.=.theta. is established. Assuming that the para-axial power of the objective optical system 100 is .beta. and the numerical aperture of the relay optical system is NAr, the expression below is established. EQU NAo=.beta..multidot.NAr (2)
The para-axial power .beta. is expressed as follows under the condition that the distortion of the objective optical system is ignored: ##EQU1## where I denotes the height of an object, I' denotes the height of an image or image height, S denotes a distance from the object, and .omega. denotes a half angle of view.
Assuming that the focal length of one of the systems of relay lenses is f and the external diameter thereof is .phi., the numerical aperture of the relay optical system 101, NAr, is provided as the expression below. ##EQU2## The focal length of one of the systems of lenses is a focal length of one front or back part of the relay optical system (that is one system of relay lenses regarded as the unit of a relay) with respect to the position of entrance-image formation. Herein, the focal length of the front part of the relay optical system is not equal to the one of the back part thereof.
When the image height I' is expressed as a ratio relative to the external diameter .phi. of the relay optical system, the following expression is given: EQU I'=n.phi.
where n denotes a ratio of the image height to the external diameter .phi..
When the focal length f of one of the systems of relay lenses is expressed as a ratio relative to the length of a relay or relay length L, the following expression is provided: EQU f=mL
where m denotes the ratio of the focal length to the relay length L.
By combining the aforesaid expressions, the angle of introversion .alpha. is rewritten as follows: ##EQU3##
Table 1 lists the values of the angle of introversion .alpha. calculated according to the expression (3) in consideration of the specifications of conventional stereoscopic-vision endoscopes. The specifications of first and second conventional stereoscopic-vision endoscopes are excerpted from Germany Utility Model Unexamined Publication No. G9302898.2. Herein, the pupil dividing ratio k is 0.75.
TABLE 1! __________________________________________________________________________ First conventional Second conventional Unit Item Symbol endoscope endoscope __________________________________________________________________________ Objective Angle of view 2.omega. 45.degree. 60.degree. optical system Distance from an object S 45.2 mm 46 mm Image height I' 3.34 mm 3.25 mm Relay optical Outer diameter .phi. 7 mm 7 mm system Numerical aperture NAr 0.1 0.1 Squared outer .phi..sup.2 /L 0.42 0.42 diameter-to-relay length ratio Outer diameter-to- .phi./L 0.06 0.06 relay length ratio Focal length-to-relay f/L 0.3 0.3 length ratio Pupil dividing Dividing ratio k 0.75 0.75 stop Whole system Angle of introversion .alpha. 1.6.degree. 1.1.degree. __________________________________________________________________________
The angle of introversion required for providing a necessary and satisfactory sense of three-dimensionality ranges from about 2.degree. to 7.degree.. When the angle of introversion ranges from 1.degree. to 2.degree., a viewer has an insufficient sense of three-dimensionality. When the angle of introversion is less than 1.degree., a viewer has almost no sense of three-dimensionality. When the angle of introversion exceeds 7.degree., a viewer has an excessive sense of three-dimensionality or fails to have a stereoscopic vision and feels fatigued.
As seen from Table 1, the angle of introversion .alpha. of the first conventional stereoscopic-vision endoscope is 1.6.degree. with the endoscope separated from an object by the best distance. A viewer has a slight sense of three-dimensionality but does not have a sufficient sent thereof. In this case, the angle of view is as small as 45.degree. and makes the endoscope unsuitable for surgical use through endoscopic observation.
The second conventional stereoscopic-vision endoscope has an angle of view of 60.degree.. The angle of introversion is as small as 1.1.degree. with the endoscope separated from an object by the best distance. With this value of the angle of introversion, the object cannot be discerned three-dimensionally in practice.
FIG. 39 is a graph of an angle of introversion, which is calculated according to the expression (3), versus a distance from an object. The angle of introversion provided by the second conventional endoscope under the condition of (.phi..sup.2 /L)=0.42 as well as the one provided thereby under the condition of (.phi..sup.2 /L)=0.82 are also indicated graphically. Assuming that an angle of introversion permitting even a slight sense of three-dimensionality is 1.degree. or more and an angle of introversion not resulting in an excessive sense of three-dimensionality is 7.degree. or less, the distance from an object that can be viewed three-dimensionally ranges from 10 to 50 mm in the case of the second conventional endoscope. Stereoscopic vision is confined to a limited space. The angle of introversion relative to the best distance from an object, 46 mm, is about 1.degree., providing substantially no sense of three-dimensionality. When the angle of view .omega. in the expression (3) is set to a small value, the angle of introversion .alpha. becomes larger. Nevertheless, the angle of view, 45.degree., provides an insufficient sense of three-dimensionality for practical use.